(1) Technical Field
The present invention relates to a propylene polymer composition and foam moldings thereof. More precisely, the invention relates to a propylene polymer composition of high melt tension especially suitable to foam molding, and relates to foam moldings of the polymer composition of uniform thickness.
(2) Related Art
Because of its good mechanical properties and chemical resistance and the fact it is economical to use, polypropylene is widely used in the art of various molding. However, as its melt tension is low, the moldability of polypropylene in, for example, blow molding, foam molding and extrusion molding is in fact not good.
For increasing the melt tension of polypropylene, for example, a method of reacting polypropylene in melt with an organic peroxide and a crosslinking promoter (JP-A-59-93711, 61-152754); and a method of reacting semi-crystalline polypropylene with a peroxide having a low decomposition point in the absence of oxygen to produce gel-free propylene having long free end branches (JP-A-2-298536)are known.
These methods are effective for increasing the melt tension of polypropylene, but are problematic in that the products often have an offensive smell as they require an organic peroxide and a crosslinking promoter. In addition, when the products are recycled, their thermal stability is lowered as the melt flow rate of polypropylene increases in the recycling step. Moreover, the methods are not fully satisfactory from a productivity standpoint.
Another method for increasing the melt viscoelasticity such as melt tension of polypropylene has been proposed, which comprises producing a composition containing polypropylene, and polyethylene or polypropylene that differs from the polypropylene in the intrinsic viscosity or the molecular weight by blending them or by polymerizing monomers in multi-stage polymerization. For example, from 2 to 30 parts by weight of ultra-high-molecular polypropylene is added to 100 parts by weight of ordinary polypropylene, and the resulting mixture is extruded at a temperature falling between the melting point of the mixture and 210xc2x0 C. (JP-B-61-28694). However, the melt tension of these compositions is still not fully satisfactory. The multi-stage polymerization to produce such an ultra-high-molecular polyolefin requires extremely low-temperature reaction, for which, therefore, the ordinary process must be specifically modified and, in fact, its productivity is inevitably lowered.
Apart from the methods mentioned above, also known are a method of producing polypropylene of high melt tension by polymerizing propylene in the presence of a prepolymerized catalyst prepared by prepolymerizing ethylene and a polyene compound with a supported-type titanium-containing solid catalyst component and an organoaluminium compound catalyst component (JP-A-5-222122); and a method of producing an ethylene-xcex1-olefin copolymer of high melt tension in the presence of a polyethylene-containing prepolymerized catalyst in which the polyethylene has an intrinsic viscosity of at least 20 dl/g and which is prepared through prepolymerization of ethylene alone with the catalyst components as above (JP-A-4-55410). In these methods, however, an additional third component, i.e. polyene compound must be prepared, and the prepolymerized polyethylene could not uniformly disperse in the final product, i.e. the polyolefin composition. Therefore, the methods are problematic in that the quality of the polyolefin composition is unstable. In addition, also known are an olefin (co)polymer composition of high melt tension which is obtained by polymerizing propylene in the presence of a polyethylene-containing prepolymerized catalyst in which the polyethylene has an intrinsic viscosity of at least 15 dl/g; and a method for producing it (WO97/14725)
However, when conventional compositions are kneaded in melt at temperatures not higher than 250xc2x0 C., their melt could exhibit high melt tension, but the thickness of their foam moldings is apt to be non-uniform and the appearance thereof is apt to be worse. Therefore, when the foam moldings are thermoformed (fabricated), they have a tendency to be readily broken at the thin part. On the other hand, when the compositions are kneaded in melt at a temperature higher than 250xc2x0 C., the thickness of their foam moldings may be uniform, but their melt tension is apt to be low. Therefore, the foam moldings have a tendency to be difficult to thermoform (fabricate), as their drawdown is great.
The present invention has been made in consideration of the situation of the prior art mentioned above. One object of the invention is to provide a propylene polymer composition of high melt tension which, even in the case that any special additive isn""t used, can be foamed into good foams of uniform thickness that can be well thermoformed (fabricated) with no drawdown trouble.
We, the present inventors have assiduously studied to solve the problems noted above, and, as a result, have found that a propylene polymer composition, prepared by melt-kneading a polymer mixture comprising a specific ethylene polymer and a specific olefin multi-stage polymer, and having specific melt properties, attains the intended object. The invention includes the following 1) to 8).
1) A propylene polymer composition obtain by melt-kneading a polymer mixture comprising an olefin multi-stage polymer (B) mentioned below and an ethylene polymer (A) mentioned below in an amount from 0.01 to 5.0 parts by weight relative to 100 parts by weight of the polymer (B),
and satisfying the following formula (1) in point of a melt tension (MS, unit:cN) at 190xc2x0 C. and melt flow rate (MFR(T), unit:dg/min) of the composition:
Log(MS) greater than 0.91xe2x88x920.23xc3x97Log(MFR(T))xe2x80x83xe2x80x83(1),
[1] ethylene polymer (A), having an intrinsic viscosity of from 15 to 100 dl/g measured in tetralin at 135xc2x0 C.,
[2] olefin multi-stage polymer (B), obtained by producing a propylene polymer (I) in a polymerization step (I) to a range of from 30 to 95% by weight of the olefin multi-stage polymer, followed by producing an olefin polymer (II) in a subsequent polymerization step (II) to a range of from 5 to 70% by weight of the olefin multi-stage polymer.
2) The propylene polymer composition of 1) above, for which the polymer mixture is melt-kneaded at a temperature thereof falling between 280 and 500xc2x0 C.
3) The propylene polymer composition of 1) above, for which the polymer mixture is prepared by producing the olefin multi-stage polymer (B) in the presence of a pre-activated catalyst prepared by supporting the ethylene polymer (A) on an olefin polymerization catalyst.
4) The propylene polymer composition of 3) above, for which a melt flow rate (MFR(i), unit:dg/min) of the polymer after the polymerization step (I) and a melt flow rate (MFR(ii), unit:dg/min) of the olefin polymer (II) satisfy the following formula (2) and (3):
3xe2x89xa6Log(MFR(i)/MFR(ii))xe2x89xa67xe2x80x83xe2x80x83(2)
MFR(ii) less than 1xc3x9710xe2x88x923 dg/minxe2x80x83xe2x80x83(3).
5) The propylene polymer composition of any one of 1) to 4) above, for which the propylene polymer (I) is a propylene homopolymer, or a propylene-olefin copolymer of propylene and an olefin with from 2 to 12 carbon atoms except propylene, having at least 50% by weight, based on the copolymer, of propylene units.
6) The propylene polymer composition of any one of 1) to 5) above, for which the olefin polymer (II) is a homopolymer or copolymer of an olefin with from 2 to 12 carbon atoms.
7) A foam molding of the propylene polymer composition of any one of 1 to 6) above.
8) The foam molding of 7) above, of which the fluctuation in the thickness falls within a range of xc2x15% of the mean thickness of the foam molding.
The propylene polymer composition of the invention is obtained by melt-kneading a polymer mixture of a specific olefin multi-stage polymer (B) and from 0.01 to 5.0 parts by weight, relative to 100 parts by weight of the polymer (B), of a specific ethylene polymer (A), and has specific melt properties.
If the amount of the ethylene polymer (A) in the mixture is smaller than 0.01 parts by weight relative to 100 parts by weight of the olefin multi-stage polymer (B) therein, the melt tension of the resulting propylene polymer composition will not increase; but if larger than 5.0 parts by weight, the melt tension increase will not further increase and, in addition, the ethylene polymer (A) and the olefin multi-stage polymer (B) will not uniformly disperse in the mixture. Preferably, the propylene polymer composition of the invention contains from 0.02 to 3.0 parts by weight, relative to 100 parts by weight of the olefin multi-stage polymer (B), of the ethylene polymer (A).
The ethylene polymer (A) has an intrinsic viscosity of from 15 to 100 dl/g, measured in tetralin at 135xc2x0 C. If the intrinsic viscosity of the ethylene polymer (A) is lower than 15 dl/g, the melt tension of the propylene polymer composition obtained by melt-kneading the polymer mixture will not increase. The uppermost limit of the intrinsic viscosity of the ethylene polymer (A) is not specifically defined. However, if the intrinsic viscosity difference between the two is too large, the ethylene polymer (A) and the olefin multi-stage polymer (B) will not uniformly disperse in the resulting mixture; and if so, the propylene polymer composition will not stably exhibit high melt tension. In consideration of the fact that ethylene polymer having a higher intrinsic viscosity requires a lower polymerization temperature, it is desirable that the uppermost limit of the intrinsic viscosity of the ethylene polymer (A) is up to around 100 dl/g from a productivity standpoint. Preferably, the intrinsic viscosity of the ethylene polymer (A) falls between 17 and 80 dl/g, more preferably between 17 and 50 dl/g.
The intrinsic viscosity of the ethylene polymer (A) must not be lower than 15 dl/g. Therefore, for efficiently increasing its molecular weight, the ethylene polymer (A) is preferably an ethylene homopolymer, or an ethylene-olefin copolymer of ethylene and an olefin with from 3 to 12 carbon atoms having at least 50% by weight, based on the copolymer, of ethylene units. More preferably, it is an ethylene homopolymer, or an ethylene-olefin copolymer of ethylene and an olefin with from 3 to 12 carbon atoms having at least 70% by weight, based on the copolymer, of ethylene units. Even more preferably, it is an ethylene homopolymer, or an ethylene-olefin copolymer of ethylene and an olefin with from 3 to 12 carbon atoms having at least 90% by weight, based on the copolymer, of ethylene units.
When the ethylene polymer (A) is an ethylene-olefin copolymer, the olefin with from 3 to 12 carbon atoms to be copolymerized with ethylene includes, for example, propylene, 1-butene, 1-pentene, 1-hexene, 1-octene, 1-decene, 4-methyl-1-pentene, 3-methyl-1-pentene. Two or more such olefins may be combined for use herein.
One and the same type or two or more different types of such ethylene polymer (A) maybe mixed with the olefin multi-stage polymer (B).
The olefin multi-stage polymer (B) which is a component of the polymer mixture for use in the invention is prepared by producing a propylene polymer (I) in a polymerization step (I) to a range of from 30 to 95% by weight of the olefin multi-stage polymer, followed by producing an olefin polymer (II) in a subsequent polymerization step (II) to a range of from 5 to 70% by weight of the olefin multi-stage polymer. The melt flow rate of the olefin multi-stage polymer (B) preferably falls between 0.1 and 100 dg/min, more preferably between 0.1 and 80 dg/min.
The propylene polymer composition of the invention is favorably obtained by melt-kneading the propylene polymer mixture at a specific temperature. The xe2x80x9cpolymer mixturexe2x80x9d defined herein is one prepared by mixing the olefin multi-stage polymer (B) and the ethylene polymer (A), and this is not specifically defined as to whether the two are in the resulting mixture to some degree of freedom from each other or they are apparently bonded to each other in the mixture. The operation of xe2x80x9cmelt-kneadingxe2x80x9d in the invention is meant to indicate that the polymer mixture is kneaded in melt under heat by the use of an extrusion granulator or the like.
The propylene polymer composition of the invention is favorably obtained by melt-kneading the polymer mixture defined herein at a temperature of the mixture falling between 280 and 500xc2x0 C., more preferably between 280 and 450xc2x0 C. If the temperature at which the polymer mixture is melt-kneaded is lower than 280xc2x0 C., the fluctuation in the thickness of the foam molding of the polymer composition will not fall within a range of xc2x15% of the mean thickness of the foam molding. If so, the appearance of the foam molding is not good, and, in addition, when the foam molding is thermoformed (fabricated), the thin part of it will be broken.
The propylene polymer composition of the invention satisfies the following formula (1) in terms of its MFR(T) and MS.
Log(MS) greater than 0.91xe2x88x920.23xc3x97Log(MFR(T))xe2x80x83xe2x80x83(1).
MFR(T) preferably falls between 0.1 and 100 dg/min. If MFR(T) is larger than 100 dg/min, the melt tension of the resulting propylene polymer composition will not increase, and if so, the foam molding of the polymer composition is apt to draw down when thermoformed (fabricated). More preferably, MFR(T) falls between 0.1 and 80 dg/min.
The uppermost limit of MS of the propylene polymer composition of the invention is not specifically defined. However, if its MS is too high, the moldability of the composition will worsen. Even more preferably, the propylene polymer composition of the invention satisfies the following formula (4):
1.2xe2x88x920.23xc3x97Log(MFR(T)) greater than Log(MS) greater than 0.91xe2x88x920.23xc3x97Log(MFR(T))xe2x80x83xe2x80x83(4).
Still more preferably, it satisfies the following formula (5):
1.15xe2x88x920.23xc3x97Log(MFR(T)) greater than Log(MS) greater than 0.91xe2x88x920.23xc3x97Log(MFR(T))xe2x80x83xe2x80x83(5).
Most preferably, it satisfies the following formula (6):
1.15xe2x88x920.23xc3x97Log(MFR(T)) greater than Log(MS) greater than 0.96xe2x88x920.23xc3x97Log(MFR(T))xe2x80x83xe2x80x83(6).
The melt tension (MS) of the propylene polymer composition referred to herein is measured as follows, using Melt Tension Tester Model 2 (by Toyo Seiki Seisakusho). A sample of the propylene polymer composition is heated at 190xc2x0 C. in a device, and its melt is extruded out into air at 23xc2x0 C. through a nozzle of 2.095 mm in diameter at a rate of 20 mm/min to form a strand. While the strand is taken up at a rate of 3.14 m/min, its tension is measured. This indicates the melt tension (unit, cN) of the strand, i.e. the propylene polymer composition.
The propylene polymer composition of the invention is favorably produced by mixing the ethylene polymer (A) and the olefin multi-stage polymer (B) in a predetermined ratio in any desired manner into a polymer mixture followed by melt-kneading it at a predetermined temperature.
More preferably, in the presence of a pre-activated catalyst prepared by supporting the ethylene polymer (A) on an olefin polymerization catalyst, a propylene polymer (i) is produced in a polymerization step (I) and then an olefin polymer (ii) is produced in a subsequent polymerization step (II) to prepare a polymer mixture that contains [1] the ethylene polymer (A) and [2] the olefin multi-stage polymer (B) composed of the propylene polymer (i) and the olefin polymer (ii), and the polymer mixture is melt-kneaded into the desired propylene polymer composition.
For the olefin polymerization catalyst to be used in the method of producing the propylene polymer composition, any and every known catalyst component consisting essentially of a transition metal compound catalyst component that contains a titanium compound heretofore proposed for polyolefin production can be used. Especially preferred is a titanium-containing solid catalyst component (c) suitable for industrial production.
For the component (c), for example, there have been proposed a titanium-containing solid catalyst, component consisting essentially of a titanium trichloride composition (JP-B-56-3356, -59-28573, -63-66323), and a titanium-containing, supported catalyst component in which titanium tetrachloride is supported on a magnesium compound and which comprises, as the essential ingredients, titanium, magnesium, halogen and electron donor (JP-A-62-104810, -62-104811, -62-104812, -57-63310, -57-63311, -58-83006, -58-138712). Any of these is employable herein.
The titanium-containing solid catalyst component (c) may be further combined with an organometal compound (a) for the olefin polymerization catalyst for use herein. The organo metal compound (a) is, for example, a compound having an organic group of a metal selected from those of Groups 1, 2, 12 and 13 of the Periodic Table (1991), and includes, for example, organo lithium compounds, organosodium compounds, organomagnesium compounds, organozinc compounds and organoaluminium compounds. Especially preferred are organoaluminium compounds of the following general formula:
AlR1pR2qX3xe2x88x92(p+q)
wherein R1 and R2 each independently represent a hydrocarbon group such as an alkyl, cycloalkyl or aryl group, or an alkoxy group; X represents a halogen atom; and p and q each are a positive integer satisfying the condition of 0 less than p+qxe2x89xa63.
The organoaluminium compounds preferred for the organometal compound (a) are, for example, trialkylaluminiums, dialkylaluminium monohalides, dialkylaluminium hydrides, alkylaluminium sesquihalides and monoalkylaluminium dihalides proposed in WO97/14725, and alkoxyalkylaluminiums. Preferred for use herein are trialkylaluminiums and dialkylaluminium monohalides. One or more of these organoaluminium compounds may be used herein either singly or as combined.
If desired, the combination of the titanium-containing solid catalyst component (c) and the organometal compound (a) may be further combined with an electron donor (e) for controlling the olefin polymer production speed and/or the polymer stereospecificity, and this may be used for the catalyst for olefin polymerization.
The electron donor (e) includes, for example, an organic compound having any of oxygen, nitrogen, sulfur and phosphorus atoms in the molecule and an organosilicon compound having Sixe2x80x94Oxe2x80x94C bond in the molecule, such as ethers, alcohols, esters, aldehydes, fatty acids, ketones, nitrites, amines, amides, urea and thioureas, isocyanates, azo compounds, phosphines, phosphites, phosphinates, hydrogen sulfide and thioethers, neoalcohols and silanols proposed in WO97/14725.
One or more of these electron donors may be used herein either singly or as combined.
Before being pre-activated by supporting the ethylene polymer (A) thereon, propylene is preferably prepolymerized with the olefin polymerization catalyst to support the resultant propylene polymer thereon. A preferred method for this comprises prepolymerizing propylene with an olefin polymerization catalyst (which comprises a titanium-containing catalyst component (c) and contains from 0.01 to 100 mols, preferably from 0.1 to 50 mols, relative to one mol of titanium in the titanium-containing catalyst component (c), of an organometal compound (a), and from 0 to 50 mols, preferably from 0 to 20 mols, relative to one mol of titanium therein, of an electron donor (e)) so that the catalyst may support propylene polymer having an intrinsic viscosity smaller than 15 dl/g, followed by supporting thereon from 0.01 to 5000 g, per gram of the titanium-containing solid catalyst component (c), of the ethylene polymer (A) having an intrinsic viscosity of from 15 to 100 dl/g to thereby pre-activate the catalyst.
Apart from this, the olefin polymerization catalyst may be first pre-activated by supporting the ethylene polymer (A) thereon, and then prepolymerized with propylene so that it can further support propylene polymer.
In the prepolymerization step and the pre-activation step, if the amount of the organometal compound (a) used is too small, the polymerization speed will be low; but if too large, the polymerization speed will not be accelerated anymore, and much of the residue of the organometal compound (a) will remain in the final product, i.e. the propylene polymer composition. If the amount of the electron donor (e) used is too large, the polymerization speed will be low.
The prepolymerization and pre-activation maybe effected in a liquid phase of an inert solvent or an olefin serving as a solvent, or may also be effected in a vapor phase not requiring a solvent. The inert solvent includes, for example, aliphatic hydrocarbons such as butane, pentane, hexane, heptane, octane, isooctane, decane, dodecane; alicyclic hydrocarbons such as cyclopentane, cyclohexane, methylcyclohexane; aromatic hydrocarbons such as toluene, xylene, ethylbenzene; and gasoline fractions and hydrogenated diesel oil fractions. If the solvent used is too much, it requires a large-scale reactor and the polymerization speed is difficult to efficiently control and maintain.
The pre-activation is not specifically defined so far as it produces from 0.01 to 5000 g, preferably from 0.05 to 2000 g, more preferably from 0.1 to 1000 g, per gram of the titanium-containing solid catalyst component (c), of the ethylene polymer (A). In general, however, it is effected at a relatively low temperature falling between xe2x88x9240 and 40xc2x0 C., preferably between xe2x88x9240 and 30xc2x0 C., more preferably between xe2x88x9240 and 20xc2x0 C. or so, under a pressure falling between 0.1 and 5 MPa, preferably between 0.2 and 5 MPa, more preferably between 0.3 and 5 MPa, for a period of time falling between 1 minute and 24 hours, preferably between 5 minutes and 18 hours, more preferably between 10 minutes and 12 hours. The pre-activation may be effected in the presence of hydrogen, but for producing the ethylene polymer (A) having an intrinsic viscosity of from 15 to 100 dl/g, it is preferably effected in the absence of hydrogen.
The pre-activated catalyst obtained in the manner as above is, if desired, further combined with an additional organoaluminium compound (axe2x80x2) and an additional electron donor (exe2x80x2), and used in producing the olefin multi-stage polymer (B). For the additional organoaluminium compound (axe2x80x2) and electron donor (exe2x80x2) optionally to be added to the catalyst, the same as those of the organoaluminium compound (a) and the electron donor (e) mentioned hereinabove can be used. One and the same type or two or more different types of these compounds may be used herein either singly or as combined. The additional organoaluminium compound (axe2x80x2) and electron donor (exe2x80x2) optionally to be added to the catalyst may be independently the same as those of the organoaluminium compound (a) and the electron donor (e) used for pre-activation.
The olefin multi-stage polymer (B) may be produced in any known olefin polymerization process. Concrete examples are a slurry polymerization method for olefin polymerization in an inert solvent of for example, aliphatic hydrocarbons (e.g., propane, butane, pentane, hexane, heptane, octane, isooctane, decane, dodecane), alicyclic hydrocarbons (e.g., cyclopentane, cyclohexane, methylcyclohexane), aromatic hydrocarbons (e.g., toluene, xylene, ethylbenzene), gasoline fractions and hydrogenated diesel oil fractions; a bulk polymerization method in which the olefin to be polymerized serves also as a solvent; a vapor-phase polymerization method of polymerizing an olefin in a vapor phase; a liquid-phase polymerization method of producing a liquid polyolefin; and a combined polymerization process of two or more of these methods.
In any of these polymerization processes, the olefin multi-stage polymer (B) is produced at a polymerization temperature falling between 20 and 120xc2x0 C., preferably between 30 and 100xc2x0 C., more preferably between 40 and 100xc2x0 C., under a polymerization pressure falling between 0.1 and 5 MPa, preferably between 0.3 and 5 MPa, in a continuous, semi-continuous or batch system that takes a period of polymerization time falling between 5 minutes and 24 hours. Under the condition as above, the olefin multi-stage polymer (B) can be favorably produced at a controlled reaction speed and its production efficiency is high. Like in known olefin polymerization, hydrogen may be introduced into the polymerization system to control the molecular weight of the olefin multi-stage polymer (B) produced.
In the propylene polymer composition of the invention, MFR(i) of the polymer after the polymerization step (I) and MFR(ii) of the olefin polymer (II) preferably satisfy the following formulae (2) and (3):
3xe2x89xa6Log(MFR(i)/MFR(ii))xe2x89xa67xe2x80x83xe2x80x83(2)
MFR(ii) less than 1xc3x9710xe2x88x923 dg/minxe2x80x83xe2x80x83(3).
MFR(i) is obtained by actually measuring the polymer after the polymerization step (I). On the other hand, MFR(ii) is derived from a melt flow rate (MFR(b)) of the polymer after the polymerization step (II), which is obtained by actually measuring the polymer; and the content fraction (W1) of the propylene polymer (I) in the olefin multi-stage polymer (B); and the content fraction (W2) of the olefin polymer (II) therein, according to the following formulae (7) and (8):
Log(MFR(b))=W1xc3x97Log(MFR(i)+W2xc3x97Log(MFR(ii))xe2x80x83xe2x80x83(7)
W1+W2=1xe2x80x83xe2x80x83(8).
If the value of Log(MFR(i)/MFR(ii)) is smaller than 3, the melt tension of the propylene polymer composition will not increase. If Log(MFR(i)/MFR(ii)) is larger than 7, the thickness of the foam molding of the propylene polymer composition will not be uniform. Regarding the formula (2), it is more desirable that MFR(i) and MFR(ii) satisfy the following formula (9):
4xe2x89xa6Log(MFR(i)/MFR(ii))xe2x89xa67xe2x80x83xe2x80x83(9).
If MFR(ii) of the olefin polymer (II) is larger than 1xc3x9710xe2x88x923 dg/min, the melt tension of the propylene polymer composition will not increase, and if so, the foam molding of the polymer composition is apt to draw down when thermoformed (fabricated). Regarding the formula (3), it is more desirable that MFR(ii) satisfies the following formula (10):
MFR(ii) less than 5xc3x9710xe2x88x924 dg/minxe2x80x83xe2x80x83(10).
MFR(i) of the polymer after carrying out the polymerization step (I) preferably falls between 0.1 and 100 dg/min, more preferably between 0.1 and 50 dg/min.
The formulae (2) and (3) that define the propylene polymer composition of the invention preferably satisfy the following formulae (11) and (12), respectively:
xe2x80x834xe2x89xa6Log(MFR(i)/MFR(ii))xe2x89xa67xe2x80x83xe2x80x83(11)
MFR(ii) less than 5xc3x9710xe2x88x924 dg/min (12).
The propylene polymer (I), which is a component of the propylene polymer composition of the invention is preferably a propylene homopolymer, or a propylene-olefin copolymer of propylene and an olefin with from 2 to 12 carbon atoms except propylene, having at least 50% by weight, based on the copolymer, of propylene units. More preferably, it is a propylene homopolymer, or a propylene-olefin copolymer of propylene and an olefin with from 2 to 12 carbon atoms except propylene, having at least 70% by weight, based on the copolymer, of propylene units. The olefin with from 2 to 12 carbon atoms except propylene is preferably ethylene or 1-butene.
The olefin polymer (II), which is a component of the olefin multi-stage polymer (B) is a homopolymer or copolymer of an olefin with from 2 to 12 carbon atoms. The olefin for the olefin polymer (II) includes, for example, ethylene, propylene and 1-butene. More preferably, the olefin polymer (II) is an ethylene-olefin copolymer of ethylene and an olefin with from 3 to 12 carbon atoms; even more preferably an ethylene-olefin copolymer of ethylene and an olefin with from 3 to 12 carbon atoms, having from 30 to 80% by weight, still more preferably from 4 to 70% by weight, based on the copolymer, of ethylene units. One example of the preferred olefin polymer (II) is an ethylene-propylene copolymer containing from 30 to 80% by weight, based on the copolymer, of ethylene units. Most preferably, the olefin polymer (II) is an ethylene-propylene copolymer containing from 40 to 70% by weight, based on the copolymer, of ethylene units.
The olefin multi-stage polymer (B) for use in the invention contains from 30 to 95% by weight of the propylene polymer (I) and from 5 to 70% by weight of the olefin polymer (II). If the content of the propylene polymer (I) in the polymer (B) is smaller than 30% by weight, the thickness of the foam molding of the propylene polymer composition is likely to be uneven and the appearance thereof is likely to be worse. If the content of the propylene polymer (I) therein is larger than 95% by weight, the melt tension of the propylene polymer composition is unlikely to increase enough. Preferably, the olefin multi-stage polymer (B) for use in the invention contains from 40 to 95% by weight of the propylene polymer (I) and from 5 to 60% by weight of the olefin polymer (II).
MFR(i), MFR(II), MFR(b) and MFR(T) defined herein are all measured at 230xc2x0 C. under a load of 2.16 kg, according to the condition 14 in Table 1 in JIS K7210.
The propylene polymer composition is especially favorable for foam molding. Its foam molding is good, as the fluctuation in the thickness thereof is well controlled to fall within a range of xc2x15% of the mean thickness of the foam molding.
In case where the polymer mixture for use in the invention is prepared in a method of producing the olefin multi-stage polymer (B) in the presence of a pre-activated olefin polymerization catalyst that supports the ethylene polymer (A) thereon, the method is optionally followed by known post treatment of, for example, catalyst inactivation, catalyst residue removal and drying, after the production of the olefin multi-stage polymer (B) therein.
For producing the foam moldings of the invention, employable is any known extrusion foaming method. For example, a foaming agent is added to the propylene polymer composition of the invention, and this is melt-kneaded and extruded out through a T-die or a circular die under reduced pressure. After having been thus directly extruded out through an extruder into sheet or cylindrical foams, they may be cut in the machine direction. The amount of the foaming agent to be added may fall between 0.1 and 10 parts by weight relative to 100 parts by weight of the propylene polymer composition of the invention.
The foaming agent may be any known volatile foaming agent or decomposable foaming agent. The volatile foaming agent includes, for example, aliphatic hydrocarbons such as propane, butane; alicyclic hydrocarbons such as cyclobutane; halogenohydrocarbons such as chlorodifluoromethane, trifluoromethane, dichlorodifluoromethane, dichlorotrifluoroethane, dichloropentafluoroethane; inorganic gases such as carbonic acid gas, carbon dioxide, air; and water. The decomposable foaming agent includes, for example, azo compounds such as azodicarbonamide; nitroso compounds such as N,Nxe2x80x2-dinitropentamethylenetetramine; and p,pxe2x80x2-oxybisbenzenesulfonylhydrazide, and citric acid. If desired, a foaming promoter such as sodium hydrogencarbonate or citric acid may be added to the foaming polymer composition for controlling the decomposition temperature and the decomposition speed of the composition and for controlling the quantity of gas to be generated and the degree of foaming of the composition. One or more of such foaming agents may be used either singly or as combined.
The polymer mixture for use in the invention may contain, if desired, various additives such as antioxidant, UV absorbent, antistatic agent, nucleating agent, lubricant, flame retardant, antiblocking agent, colorant, and inorganic or organic filler, as well as any other different synthetic resins.